Why are the colours of medium efficiency bag filters different for each type of efficiency?

Why do we see different colored bag filters? What does the color of a medium-efficiency bag filter represent? The color of the bag filter represents its efficiency, and the colors are white, green, pink, light yellow, and white. Efficiency from F5-F9, we can see from the color what the corresponding efficiency is, the following are the corresponding colors of efficiency F5-F9: 

F5 (EN 799), 40-60% (white);

F6 (EN 779), 60-70% (green);

F7 (EN 779), 75-85% (pink);

F8 (EN 779), 85-95% (light yellow);

F9 (EN 779), 99% (white);